Halftoning is performed to render continuous tone (contone) digital grayscale and color images into patterns of pixels that can be rendered on bi-level devices such as printers. The rendered images are commonly referred to as halftone images. The pixels of a halftone image are arranged in patterns such that the halftone image is perceived as having continuous tones when viewed through the human visual system.
As the print quality of printers increases, there is an increasing threat that these printers can be utilized in illegal or fraudulent operations. For example, these printers can be used to print counterfeit currency and other counterfeit prints (e.g., hard copies of copyrighted artwork and images).
Images may be watermarked, whereby information is added to or embedded in the images. The watermarks may be visible or imperceptible. The watermarks could be used to thwart counterfeiting operations. The standard watermark on a currency is a mark that is visible only when looking through (as opposed to at) the paper. This watermark will not survive any of the standard copying processes. Nevertheless, due to the good quality of the plain content counterfeit copies of currency will usually pass for good money.
The watermarks should be robust to non-destructive image processing procedures. For instance, if a watermark is added to a continuous tone image, and the watermarked continuous tone image is then halftoned, the watermark in the halftone image could be degraded.
The embedding process should not adversely affect the quality of the halftone images. Removing a good digital watermark would significantly degrade the halftone image, and would be easily noticeable.
The halftone watermarking may be performed during dither halftoning. However, the watermarking is typically performed independent of and separate from the halftoning process.
There is a need for a watermark that is robust to non-destructive image processing, and that significantly degrades image quality when removed. There is a need for a halftoning process that can add watermarks without adversely affecting the quality of halftone images.